1. Technical Field
The present invention relates to signal processing circuits and methods for reliably detecting audible alarm signals generated by smoke detectors, carbon monoxide detectors, and/or other types of detectors.
2. Description of the Related Art
The presence of smoke, fire, hazardous carbon monoxide concentrations are commonly sensed in commercially available products using several types of technologies. These products traditionally alert the occupants using loud audible alarms of loud tones. Regulations exist that require audible alarms to produce signals that alert individuals in case of a fire and other emergencies. Examples of such regulations are: Underwriters Laboratories code 217 (UL 217, “The Standard of Safety for Single and Multiple Station Smoke Alarms”), Underwriters Laboratories code 464 (UL 464, “The Standard of Safety for Audible Signal Appliances”), Underwriters Laboratories code 1971 (UL 1971, “The Standard for Signaling Devices for the Hearing Impaired”), and Underwriters Laboratories code 2034 (UL 2034, “The Standard of Safety for Single and Multiple Station Carbon Monoxide Alarms”).
According to these and other standards, typical smoke, fire, and carbon monoxide detectors produce a 3100-3200 Hz pure tone alert signal with the intensity (or power) of 45 to 120 dB (A-weighted for human hearing). However, using the pure tone signal can result in a less than effective means of alerting people with hearing loss, older adults, children, heavy sleepers, and intoxicated persons. Traditional means of alerting can be inadequate especially at night time when the audible alarm is not located near the individual. If individuals are not alerted, the results can range from inconvenient to deadly.
Current state of the art attempts to address these deficiencies by interconnecting alarms using wires or wirelessly. Alarms thusly networked together can communicate detection conditions and “trigger” each other. One example of such technology is Kidde Wireless Smoke Alarm Model RF-SM-DC or RF-SM-AC Smoke Alarm. However, such solutions can be expensive and unreliable for alerting people with hearing loss, older adults, children, heavy sleepers, and intoxicated persons.
Another solution for these deficiencies is the introduction of a detection device that “listens” for audible signals generated by smoke, fire, and carbon monoxide detectors. Upon detecting an audible alert signal, the detection device can trigger supplemental alert devices that are configured to reliably alert the individual to the presence of fire, smoke, carbon monoxide, and the like. One type of supplemental alerting is to generate a loud low frequency (e.g., 520 Hz) square wave sound pattern that can reliably alert people with hearing loss, older adults, children, heavy sleepers, and intoxicated persons as is described in U.S. Pat. No. 6,658,123 to Crutcher, the disclosure of which is hereby incorporated by reference.
One difficulty with designing such detection devices is to reliably detect audible alert signals generated by smoke, fire, and carbon monoxide detectors. Ideally, reliable detection cannot tolerate any false negative conditions (i.e., non-detection of actual audible alert signals) and can tolerate only a few false positive conditions (i.e., false detection of audible alert signals when, in fact, none actually occurred). These can be restated as requirements of high sensitivity and high specificity, respectively. False negative conditions cannot be tolerated because if individuals are not alerted, the results can range from inconvenient to deadly. False positive conditions ought to occur infrequently because they can cause individuals to disconnect smoke, fire, and carbon monoxide detectors and/or the detection devices.
Even though existing regulations (such as UL 217, UL 464, UL 1971, UL 2034, NFPA 72, ANSI 53.41, and ISO-8201) govern the characteristics of audible alert signals generated by commercially available smoke, fire, and carbon monoxide detectors, there still exist many problems with the reliable detection of these signals. Testing performed by LifeTone Technology™ demonstrates that commercially available detectors often produce audible alert signals that “drift” from the specifications of the existing regulations. In fact, approximately 11% of commercially available detectors do not meet the specifications of some or all of the above mentioned UL regulations. In addition, sources of audible alert signals can be located far away causing audible alert signals to have weak intensity by the time the detection device is reached. Moreover, other signals such as music, television programming, and noise can distort, weaken, or drown out audible alert signals before the detection device is reached. These problems can be further compounded when background signals have components, which potentially have higher signal intensity than the audible alert signals, and when such components also overlap in frequency and periodicity with the audible alert signals. Accordingly, there is a need for a detection device that can provide reliable detection (i.e., high sensitivity) while producing few false negative conditions (i.e., high specificity).